This invention pertains to an improved method for controlling color bleed in a color-image processing setting of the category wherein color error diffusion is performed with respect to an output imaging device, such as a printer. In particular this invention relates to such a setting in which the output device is a bi-tonal device that effects (prints, in the case of a printer) bi-tonal color images in a defined color space. The output color space of such device is referred to herein as a device output color space, and a preferred and best-mode manner of implementing the invention is illustrated and described herein representationally with regard to a halftone color printer which operates in C, M, Y, K color space. Additionally, while it will become readily apparent that the present invention can be invoked with significant utility in a setting wherein any form of color error diffusion takes place, the mentioned preferred manner of practicing the invention has special utility where color vector error diffusion is the type of diffusion which is performed, and thus, the invention is specifically described herein in such an environment.
Color bleed, or bleeding, is a common problem in color-image processes which involve color error diffusion. If, in an image, there is a very similar color throughout a local region, sometimes the accumulated color error which arises in the context of error diffusion cannot be digested in this region, and this situation can lead to color bleeding to a next-adjacent color region, and thereby produce a dramatic and unwanted mix of color in that region. While color bleeding can sometimes be a problem in approaches which do not use vector color error diffusion, color bleeding can become especially severe, and very unacceptable, in the case of vector color error diffusion of the kind that uses a device-independent color space for halftone input space, and which combines that practice with the additional practice of performing device color-conversion in the dithering process.
The typical prior art approach used to minimize color bleeding employs pre-adjustment with respect to the input data image. That approach requires an extra processing step for implementing error diffusion, and given this, and with regard to the issue of efficiency of performance, the approach is not optimal. The present invention disclosed herein uses a simple and efficient method to avoid color bleeding, and implements this method in a manner which not only essentially completely obviates the color-bleeding problem, but also optimizes the efficiency of performance.
Accordingly, the present invention, to address the color-bleed issue, takes the unique, simple and extremely effective approach of utilizing only a portion (a reduction), rather than the whole, of an accumulated diffusion error amount, and applies this reduction in accordance, as will be described, with the specific nature of an employed error diffusion filter. In implementing this practice of the invention, I have found that it is preferable, where possible, to base the proposed reduction amount of distributed accumulated diffusion error on a practice using a “power-of-2” approach which neatly fits into the otherwise conventional bit-wise character of the normal digital-computer world, and which allows a selected reduction to be handled very simply by the conventional operation of a shift register. I have also determined that, while various different kinds of diffusion error filters may be employed, and several are specifically described herein, an especially satisfactory filter for most applications is the well-known Floyd and Steinberg filter. I have further determined that a very satisfactory, and indeed a preferred, proportion of the total calculated accumulated diffusion error to use is 15/16. Other proportions can, of course, be chosen, but this particular one appears to provide the most strikingly appealing results. It turns out that this proportion not only works very well, but also (a) readily enables the “power-of-2” approach mentioned above, and (b) fits extremely well, as will be seen in the discussion below, in a situation where a Floyd and Steinberg filter is employed.
No matter what filter is employed, and no matter what specific character of error diffusion is used, color bleed can be substantially fully eliminated by the described practice of the present invention—a practice involving intentional use of some proportioned amount which is less than the full calculated amount of diffusion error in each instance involving pixel distribution of such error.
These and various other features and advantages which are attained by practice of the invention will become more fully apparent as the detailed description which now follows is read in conjunction with the accompanying drawings.